The present invention relates to a dielectric filter having a plurality of resonant conductors arranged in parallel relative to each other, and a method of adjusting a coupling capacitance of such a dielectric filter.
There is known a dielectric filter comprising a dielectric block and two or more resonant conductors arranged in parallel with each other in the dielectric block and formed by coating the inner surface of respective through bores of the dielectric block with an inner conductive film, the outer surfaces of the dielectric block being coated by an outer conductive film except one of the end surfaces carrying the openings of said through bores, making the end surface open circuited.
Japanese Patent Publication No. 3-69202 discloses a dielectric filter having a configuration as illustrated in FIG. 1 of the accompanying drawings. Referring now to FIG. 1, a conductor film P for a capacitive coupling circuit having a certain pattern is provided on an open-circuit end surface of a dielectric block A carrying no outer conductive film. The conductor film P of the capacitive coupling circuit includes two patterned conductors D which are connected to respective inner conductive films of resonant conductors B and separated by a gap S to provide a capacitive coupling. With this arrangement, the resonant conductors B are connected with each other by way of a coupling capacitance for an interstage connection in order to produce a filter circuit.
For directly and electrically connecting the resonant conductors B and the respective patterned conductors D of the circuit pattern, the resonant conductors B are typically made to have a length equal to a quarter of the resonance frequency or 1/4.lambda.. However, as the resonant conductors B are connected to the patterned conductors D, their inner conductive films are extended at the open-circuit end surface to substantially change the resonating lengths of the conductors B and hence the resonance frequency so that the completed dielectric filter would not perform satisfactorily. If the gap S between the patterned conductors D is modified to regulate the coupling capacitance by either removing the edges of the conductors or adding additional conductor pieces to them, the effective resonating lengths of the resonant conductors B are incidentally modified to consequently change the resonance frequency. Thus, the attempt to obtain a satisfactory dielectric filter would also be thwarted.
It is therefore the object of the present invention to provide a dielectric filter adapted to show a specified coupling capacitance without inadvertently affecting the resonance frequency.
Another object of the present invention is to provide a method of adjusting a coupling capacitance of such a dielectric filter.